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12 Propulsion Technologies That Will Increase Future Cars' Efficiency

We're about to see more changes in the coming decades since the transition from the horse to the automobile.

By
Sam Abuelsamid

Sep 30, 2016

For most of the 130 years that the hydrocarbon-fueled, reciprocating-piston internal-combustion engine (ICE) has been around, it has held an effective monopoly on automotive propulsion. Steam and electric power weren't true competitors for very long after the turn of the 20th century. In fact, ICE has been so dominant that, if you add up the cumulative sales of all the alternatives, they still only amount to a tiny fraction of the global sales of ICEs in a single year. The ICE has maintained its leadership position by virtue of its high power density, practicality, and low cost; and for those reasons, it's not going away any time soon. However, the practical realities of the world we live in will drive more changes in the coming decades than we have seen since the transition from the horse to the automobile.

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1. 48-Volt Electrical Systems: Electric Accessory Drives

The practical realities of global manufacturing and refueling infrastructure investment mean that the internal-combustion engine (ICE) will probably be the dominant powerplant for several more decades. One of the key technologies that will help ICEs become more efficient is the 48-volt electrical system. Most of today's engines drive alternators, coolant pumps, oil pumps, and air-conditioning compressors mechanically via a belt or directly, and they generally run continuously. The common automotive 12-volt electrical systems max out at 2 to 3 kW of electrical power and, with today's widespread use of electronic engine, chassis, and body controls, plus creature comforts and infotainment features, nearly all of it is used. As ever more electrical features are added, such as semi- or fully autonomous systems, more electrical power will be needed. Stepping up to the 10-kW output enabled by 48-volt generators will allow those new features and today's mechanical accessory drives to go electric. That will reduce the parasitic loads on engines and potentially boost fuel economy by about 10 percent. The insanely complex $231,825 Bentley Bentayga and the Audi SQ7 TDI, with its electric supercharger, have 48-volt electrical subsystems today, but it's expected that this will spread to the mainstream in the coming years.

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2. 48-Volt Electrical Systems: "Mild Hybrids"

Early attempts at so-called mild hybrid systems, notably from General Motors, were largely met with yawns from the car-buying public. By combining lower-voltage batteries and weaker motors than what Toyota installed in a Prius, these systems managed to achieve far less of an efficiency gain while still costing nearly as much. Engineers now expect the advent of 48-volt electrical systems to enable new mild hybrids that achieve 70 percent of the improvement from a Toyota-style hybrid at only 30 percent of the cost. Since those earlier mild hybrids operated at 100 to 170 volts, they still required all the extra safety features of any system that exceeds 60 volts. With the 48-volt electrical architecture, new mild hybrids can use thinner-gauge wiring and lower-cost connectors while still providing some regenerative braking, electric power boost, and expanded auto stop-start capability.

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3. Advanced Cylinder Deactivation

Many automakers know the perils of being first to market when the technology really isn't ready. For the 1981 model year, Cadillac introduced an innovative new V-8 engine variant that could automatically switch off two or four cylinders. Paired with throttle-body fuel injection and exceedingly crude electronics, the V-8-6-4 was far from a success. Today, cylinder deactivation is commonplace and generally imperceptible to the driver. In the coming years, additional sensors and sophisticated control algorithms will increasingly enable engines to run more of the time on less than their full complement of cylinders. By the end of this decade, GM and Delphi, licensing technology from the startup Tula Technology, expect to have V-8 engines that have advanced variable cylinder deactivation. This means that both the number of cylinders disabled—leaving as few as two active in light-load conditions—and which specific ones aren't firing will vary continuously, depending on operating conditions. According to GM, this could cut fuel consumption by as much as 15 percent.

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4. Fuel-Cell Plug-In Hybrids

It seems as though hydrogen fuel-cell technology has been claimed to be ten years away for the past 20 years. With the recent launch of theToyota Mirai and the Honda Clarity coming later this year, plus the Hyundai Tucson Fuel Cell for lease to California residents, perhaps the hydrogen-fueled electric vehicle will finally arrive. For now, the fuel cell remains expensive, and the refueling infrastructure is still patchy. One possible solution to this problem was first shown by Ford nearly a decade ago in its Airstream concept, which used a plug-in-hybrid layout to try to achieve a better balance of cost and usability. It was followed up with drivable prototype of the powertrain installed in an Edge crossover. But the gist is this: Fuel cells work most efficiently when outputting a constant stream of power. They need a battery or a capacitor to act as a buffer and handle the unavoidable fluctuating power demands of real-world driving. Using the fuel cell as a range extender could potentially achieve greater overall efficiency with zero tailpipe emissions. (Conceptually, it's similar to the Chevrolet Volt, with a fuel cell replacing the Volt's gasoline engine.) Although Ford only built one of its Edge demonstrator vehicles, Mercedes-Benz recently announced that it will produce a plug-in fuel-cell version of the GLC crossover, the GLC F-Cell, in 2017.

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5. Conformable Hydrogen Storage Tanks

Besides the cost of the fuel-cell stack and the lack of a refueling infrastructure, the other major issue with this alternative to the battery-powered car is storing the hydrogen. Until now, fuel-cell-powered vehicles have stored hydrogen gas in bulky carbon-fiber-wrapped cylinders at pressures up to 10,000 psi. For several years the U.S. Department of Energy's Advanced Research Projects Agency–Energy (ARPA-E) has been funding a variety of research projects to develop conformable storage systems for compressed natural gas. One in particular could be adapted to hydrogen storage. The "intestinal" storage system developed by San Francisco startup Volute consists of a series of what are essentially small gas cylinders linked together in series and wrapped back and forth on top of one another. An arrangement like this provides much greater packaging flexibility than one or two large cylinders, making it practical in smaller cars and in irregularly shaped areas.

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6. Silicon Anode Batteries

The battery-electric vehicle is almost as old as the car itself. Unfortunately, even with the advances made in recent decades with electrochemical batteries, their energy density pales in comparison to liquid fuels. The 960-pound, 60.0-kWh battery for the upcoming Chevrolet Bolt EV contains the same energy as just 1.8 gallons of gasoline, since a gallon of gas contains roughly 33.7 kWh of energy. Therefore, a 60.0-kWh battery pack is carrying the equivalent of 1.8 gallons of gas. The engineers and scientists haven't given up, however, and there are some very promising technologies on the horizon, including silicon anodes in the battery. Silicon can absorb a lot more electrons for a greater energy density than current chemistries that consist mostly of nickel, manganese, and cobalt. Unfortunately, absorbing all those charged particles makes the electrode swell, so pure silicon anodes are out of the question for car batteries. However, companies including Nissan are developing blends of silicon that could boost capacity by a claimed 10 to 40 percent within the next five to 10 years.

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7. Camshaft-Less Engines

To convert heat energy to mechanical work, an engine must efficiently pump air (specifically, oxygen) and fuel into the cylinders and purge exhaust gases. However, the optimal timing for when the intake and exhaust valves should open and close varies widely based on engine speed and load. Modern variable valve lift and timing and cam phasing have worked wonders in getting closer to optimal flows, but there are limits to what you can do with the classic camshaft, and it's difficult to vary the timing across individual cylinders. Ideally, each valve would be managed by its own actuator, perfectly timed for what is happening on that cylinder and crankshaft position. While electromechanical and hydraulic solenoids have been used in research labs to simulate different cam profiles before machining parts, these consumed too much power to be beneficial in production, not to mention that they were rather noisy in operation. The shift toward 48-volt electrical systems and downsized engines with fewer cylinders and valves could make the camless engine a reality in coming years. Lotus Engineering first demonstrated a camless engine in the late 1980s, and supercar maker Koenigsegg showed off its FreeValve prototype engine in early 2016.

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8. Turbine Plug-In Hybrids

Batteries are showing tremendous promise for electrification of cars and smaller light trucks, yet when payload is important, batteries are just too bulky to be practical. For bigger cargo haulers ranging from package delivery to trash collection and long-haul trucking, batteries consume too much of the payload capacity and take too long to charge to be used alone. Early Tesla team member Ian Wright's company, Wrightspeed, has devised a plug-in-hybrid powertrain for big haulers that contains enough battery for nearly 30 miles per charge and a turbine powering a range-extending generator. Recent startup Nikola Motors has a similar system for big rigs, designed to run on compressed natural gas. The cool thing about turbines is that they can run on pretty much anything that will burn, and they are very efficient when running at constant high loads, letting the battery handle acceleration and recapturing braking energy.

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9. Electric Superchargers

The turbocharger is another classic piece of engine technology that has received a new lease on life in recent years, thanks to electronic controls and high-pressure direct fuel injection. Despite the advances in design and controls, basic physics dictates that exhaust-driven turbos will almost always be subject to some lag, and underhood plumbing is a complicating factor as well. However, replacing the exhaust-driven turbine with an electric motor is the next big wave in engine downsizing. The response of an electrically driven compressor can be almost instantaneous, because a battery can spin it to eliminate lag. With an electric supercharger, an engine with cylinder deactivation can keep fewer cylinders firing, while more air is pushed into those that are left to provide the torque needed to climb a grade. When a vehicle is going downhill, the air flowing through the compressor can drive the motor to send energy back to the battery. Audi's SQ7 TDI is the first production vehicle with an electric supercharger, and we expect more to follow.

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10. Solid-State Batteries

The electrochemical battery continues to be a troubled energy storage source for electric vehicles, thanks to its stubbornly low energy density. Significant progress has been made with each jump in chemistry from lead-acid to nickel-metal hydride to lithium-ion. Nonetheless, as mentioned earlier, a cutting-edge 60.0-kWh lithium-ion battery today that weighs nearly 1000 pounds has the same energy capacity as just 1.8 gallons of gasoline. One of the limiting factors in battery performance is the liquid or gel electrolyte that sits between the electrodes. It's not the greatest conductor, and if the electrodes short, the electrolyte can be highly combustible. Enter the solid-state battery with a crystalline electrolyte material that passes electrons faster and never degrades. These batteries could last longer with greater density at lower cost. Or at least they have that potential, once the researchers from Sakti3 (recently purchased by the U.K.'s Dyson), Samsung, Google, and other companies figure out how to make more than one cell at a time.

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11. Homogeneous Charge Compression Ignition (HCCI)

In the hunt for both improved efficiency and reduced emissions, researchers have tried to develop new combustion strategies that combine the best characteristics of both the Otto (gasoline engine) and diesel cycles. This is called homogeneous charge compression ignition (HCCI). An engine with the HCCI combustion mashup would run on gasoline, while using the heat generated by compressing the incoming air to ignite the fuel without using a spark plug, like a diesel. HCCI engines should be able to improve fuel efficiency by 15 percent or more without the complex and expensive exhaust aftertreatment that diesels require. When automakers, including General Motors and Daimler, were demonstrating HCCI engines in the late 2000s, they projected that these engines might be available about now. Although work continues, this technology remains at least several years away.

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12. Variable Compression Ratio

When maximum torque is needed from an engine, a higher compression ratio can be just the ticket. Unfortunately, maximum output is rarely needed, and when a vehicle is operating at lighter loads, high compression diminishes efficiency. Just as modern electronic controls have enabled other parameters to be continuously varied, compression ratios can also be adjusted based on fluctuating engine operating conditions. Early test-lab work on variable compression ratios typically used complex mechanical systems to adjust the piston's motion relative to the crankshaft. That approach added weight and friction, which offset some of those benefits. The expanding operating range of variable cam phasing and valve lift has enabled a variable compression ratio based on keeping the intake valve open longer (to reduce the effective compression ratio)—this is also known as the Atkinson cycle—or closing it sooner (to increase it) during the compression stroke. If camless engines become a reality, the adjustments could even be made on a per-cylinder basis.

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